1. the Invention
The resent invention relates to a method of recording and reproducing an electron microscope image, and more particularly to a method of recording electron microscope images with high sensitivity and of reproducing the recorded electron microscope images in the form of electric signals in order to allow the images to be processed in various ways.
2. Description of the Prior Art
There are known electron microscopes for obtaining a magnified image of a specimen by deflecting a beam of electrons transmitted through the specimen with an electric or magnetic field. As is well known, the electron beam having passed through the specimen forms a diffraction pattern on the rear focal plane of the objective lens, and the diffracted beams interfere with each other again to produce the magnified image of the specimen. The magnified specimen image can be observed as a transmission image by projecting the image onto a screen with a projector lens. Alternatively, the rear focal plane of the objective lens may be projected for enabling the user to observe the magnified diffraction pattern of the image. Where an intermediate lens is positioned between the objective lens and the projector lens, the magnified transmission image or the diffraction pattern may be produced selectively as desired by adjusting the focal length of the intermediate lens.
For observing the magnified image or the diffraction pattern (hereinafter referred to collectively as a "transmitted electron-beam image"), it has been the general practice to place a photographic film on the image formation plane of the projector lens for exposure to the transmitted electron-beam image. According to another design, an image intensifier is employed to amplify the transmitted electron-beam image for projection. The use of photographic films is however disadvantageous in that their sensitivity to electron beams is low and the process of developing the films is complex. The image intensifier also has drawbacks in that the images produced thereby have poor sharpness and are likely to become distorted.
Transmitted electron-beam images are often processed to make them easier to see. Specifically, the transmitted electron-beam images are subjected to various signal processing modes such as tone processing, frequency emphasis, density processing, subtractive processing, and additive processing. The images are also processed to reconstruct three-dimensional images by Fourier analysis, digitize the images, and measure particle diameters. The diffraction patterns are also processed to analyze crystal information and find lattice constants, dislocations, and lattice defects. For such image and diffraction pattern processing, it has been customary to convert the electron microscope image on a developed photographic film into an electric signal with a microphotometer, convert the electric signal into a digital signal, and then process the digital signal with a computer. This process has proven unsatisfactory since it is quite complex.
In view of the conventional drawbacks, the applicants have proposed a novel method of recording and reproducing electron microscope images with high sensitivity and image quality, the method being capable of directly generating electric signals representing the electron microscope images so as to permit these images to be processed in various ways (see Japanese Patent Application No. 59(1984)-214680 corresponding to U.S. Ser. No. 786,080). Basically, this method comprises the steps of storing the energy of an electron beam transmitted through a specimen on a two-dimensional image sensor kept in vacuum, applying light or heat to the two-dimensional image sensor to cause it to discharge the stored electron beam energy therefrom as light, photoelectrically detecting the discharged light to thereby produce an electric image signal, and reproducing the transmitted electron beam image of the specimen from the electric image signal.
The two-dimensional image sensor preferably comprises a stimulable phosphor sheet as disclosed in U.S. Pat. Nos. 4,258,264; 4,276,473; 4,315,318; 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, for example. Certain phosphors, when exposed to a radiation such as an electron beam, store a part of the energy of the radiation. When the phosphor exposed to the radiation is exposed to stimulating rays such as visible light, the phosphor emits light (stimulated emission) in proportion to the stored energy of the radiation. Such a phosphor is called a stimulable phosphor. The two-dimensional image sensor is generally composed of a support and a phosphor layer disposed on the support. The stimulable phosphor layer may be formed by dispersing the stimulable phosphor in a suitable binder. However, the stimulable phosphor layer may itself be a stimulable phosphor sheet if it is self-supporting. Examples of stimulable phosphors which the stimulable phosphor sheet can be made of are described in Japanese Patent Application No. 59(1984)-214680 referred to above.
The two-dimensional sensor may also be in the form of a thermoluminescent phosphor sheet as disclosed in Japanese Patent Publication Nos. 55(1980)-47719 and 55(1980)-47720, for example. The thermoluminescent phosphor sheet emits stored radiation energy as thermoluminescence when heat is applied to the sheet. The thermoluminescent phosphor sheet may be constructed in the same manner as the stimulable phosphor sheet.
The two-dimensional image sensor is placed on the image formation plane of the electron microscope, and the electron microscope image is recorded on the two-dimensional sensor by the electron beam transmitted through the specimen. Then, the two-dimensional sensor on which the electron microscope image is stored is scanned in X and Y directions, i.e., two-dimensionally, by stimulating rays such as visible light or heat to enable the image sensor to emit the stored electron beam energy as light. The emitted light is then photoelectrically read by a suitable photoelectric transducer which produces an electric signal indicative of the transmitted electron-beam image. The electric image signal thus generated may be employed to display the electron microscope image on a display unit such as a CRT, or to record the electron microscope image permanently as a hard copy, or to store the electron microscope image temporarily on a recording medium such as a magnetic tape, a magnetic disk, or the like.
The electron microscope images can be recorded with high sensitivity using the two-dimensional image sensor of the type described above. Damage to the specimen can be reduced since the amount of the electron beam to which the specimen is exposed can be reduced. The electric image signals produced from the two-dimensional image sensor can easily be processed in various modes, such as tone processing and frequency emphasis, for example. The processing of diffraction patterns, and image analyses such as the reconstruction of three-dimensional images and image digitization can simply and quickly be performed by applying the electric signal to a computer.
Where the aforesaid proposed method is employed to record and reproduce electron microscope images, however, the high recording sensitivity is oftentimes responsible for the generation of a spot-like image or the reproduction of an electron microscope image together with fog arising most probably from a scattered electron beam or a secondary X-ray.